The present invention relates to a thin film electroluminescent (EL) device drive circuit which drives a thin film EL device in an EL display apparatus or an EL printer head by means of controlling the switching operation of thin film transistors (TFTs) which constitute the thin film EL drive circuit. More particularly, the invention relates to a thin film EL device drive circuit and a method of driving the same, which allow a high voltage pulse drive to cause the thin film EL device to emit high luminance light.
FIG. 1 is a circuit diagram showing an example of a conventional thin film EL device drive circuit in which thin film transistors are subjected to a switching operation. The circuit of FIG. 1 operates to drive a thin film electroluminescent element corresponding to one picture element. FIG. 2 is a schematic diagram showing an example of a electroluminescent display device (hereinafter referred to as an EL display device) in which a plurality of EL elements are arranged in a matrix form. FIG. 3 is a schematic diagram showing an example of a printer head in which a plurality of EL elements are arranged in an array, and one electrode of each EL element is connected commonly to a high voltage signal source Va for driving the EL elements.
The circuit of FIG. 1 will be described in more detail.
As shown in FIG. 1, the circuit includes a thin film transistor serving as a first switching element Q1, a storage capacitor Cs, a thin film transistor serving as a second switching element Q2 and an electroluminescent (EL) element C.sub.EL. In FIG. 1, one terminal of the storage capacitor Cs is connected to a source terminal S1 of the first switching element Q1 whereas the other terminal of the storage capacitor Cs is connected to the ground. A gate terminal G2 of the second switching element Q2 is connected to both one terminal of the capacitor Cs and the source terminal S1 of the first switching element Q1. A source terminal S2 of the second switching element Q2 is connected to the ground whereas a drain terminal D2 thereof is connected to one terminal of the EL element C.sub.EL, the other terminal of which is connected to a power source Va for driving the EL element C.sub.EL.
The first switching element Q1 turns on in response to a switching signal SCAN which is applied to the gate terminal G1 thereof so that the storage capacitor Cs is charged in accordance with an electroluminescent signal DATA. The first switching element Q1 is turned-off to thereby discharge the charge of the capacitor Cs. On the other hand, the second switching device Q2 is turned on when a discharge voltage of the storage capacitor Cs is applied to the gate terminal G2, to thereby causes the EL element C.sub.EL to emit a light with actuation by the power supply Va.
The EL display device will be described in detail with reference to FIG. 2 where m.times.n thin film EL device drive circuits are arranged in the matrix form. The gate terminal of the first switching element Q1 of each drive circuit arrayed in a horizontal direction is connected to a switching signal line Y to receive a switching signal SCAN. The drain terminal of the first switching element Q1 of each drive circuit arrayed in the vertical direction is connected to an information signal line X to receive an electroluminescent signal (data signal) DATA. The power supply Va is coupled to one end of the EL element C.sub.EL and one end of the capacitor Cs is connected to the ground level. This display device was proposed--in 20-lpi Electroluminescent Display Panel by T.P.Brody, F.C. Luo, et al. in "IEEE Trans. Electron Devices. Vol. ED-22, No. 9, Sept. 1975, Pages 739-749".
The construction of the EL printer head in which the EL device drive circuits are arranged in the form of array will be described with reference to an equivalent circuit of the EL printer head shown in FIG. 3 and a schematic plan view of the EL printer head shown in FIG. 4.
As shown in FIGS. 3 and 4, provided on an insulate substrate 1 made of glass and the like are an EL element array 2' including plurality of EL elements 2, a switching devices 3 which each include a thin film transistor (TFT) 3a provided corresponding to the EL element 2 and a TFT 3b for controlling the gate of the TFT 3a, a switching device array 3' including the switching elements 3 the number of which corresponds to the number of the EL elements 2 in the EL device array 2', a matrix circuit 4 which has a matrix type wiring structure for connecting the wiring from the switching device array 3' to driver ICs, a data driver IC 5 which supplies the EL control data signal DATA to the drain electrode of TFT 3b of each switching element 3 through the matrix circuit 4, and a strobe driver IC6 which supplies a block control signal STROBE to the gate electrode of TFT 3b of said switching device 3.
The operation of the EL printer head will be described with reference to FIG. 3.
Connected to each of the TFTs 3a constituting the switching element array 3' is the TFT 3b so that a plurality of EL elements C.sub.EL form blocks each of which is driven by the data driver IC5 and the strobe driver IC6.
More specifically, the TFTs 3b which are connected to the gate electrodes of the TFTs 3a commonly shares a gate line which connects to the gate electrodes of TFTs 3b so that the TFTs 3b are above to be simultaneously made conductive. The strobe driver IC6 is connected to the gate line to apply a block control pulse signal STROBE to the TFTs 3b. The drain electrode of each TFT 3b is connected to a data line to which the data driver IC5 is connected so that an EL control data signal DATA from said data driver IC5 is supplied to the drain electrodes of TFTs 3b.
With the circuit shown in FIG. 3, when the pulse signal STROBE is applied to the gate electrodes of the TFTs 3b in one of the blocks, all of the TFTs 3b are rendered conductive to thereby allow the connection of the gate electrodes of the TFTs 3a and the data line to charge the storage capacitor Cs according to the pulse signal from the data driver IC5 through the data line. Then, as a result of the TFTs 3a being turned on due to the charge of the capacitor Cs, the EL elements C.sub.EL to which the power supply voltage Va is applied emit a light. This was also disclosed in "The Construction and Characterization of a 400-dpi Thin-film Electroluminescence Edge Emitter" D. Leksell et al, Proceedings of the SID. Vol. 29/2, 1988. p147-p150.
In the above-mentioned conventional thin film EL device drive circuit, a sine wave drive circuit is employed as a high voltage signal source Va. However, in view of the luminance characteristics with the EL drive waveform, it is preferable to employ a pulse signal drive circuit in order to carry out high luminance electroluminescence, which is capable of instantaneously supplying a voltage. But, with such a pulse signal drive circuit, a large drive current capacity is required to obtain a sufficient rise characteristic of a high voltage signal for driving all EL devices load in a short period of time, resulting in preventing miniaturization of the signal source and reduction of power consumption.